Non-oriented electrical steel sheets are important parts required to change electrical energy into mechanical energy in electrical devices. For the reduction of energy, changes in the magnetic properties of the steel sheets, i.e., a reduction in iron loss and an increase in magnetic flux density are required. The iron loss means energy lost as heat in a process of energy transformation, and the magnetic flux density is expressed as force that generate power. If the iron loss is low, energy loss can be reduced, and if the magnetic flux density is high, the copper loss of electrical devices can be reduced, thus making it possible to reduce the size of the electrical devices.
To manufacture a material having low iron loss and high magnetic flux density, the texture of the final annealed steel sheet needs to be improved, and the texture improvement is greatly influenced by component design and hot rolling. Thus, the development of a proper component system and the optimization of hot rolling conditions are needed.
For this purpose, in a conventional manufacturing process, the annealing of hot-rolled sheets is performed to homogenize the texture of hot-rolled sheets and to make grain size coarse. However, the process of annealing the hot-rolled sheets acts as a main cause of cost increase resulting from the addition of processes. Recently, as demand for electrical steel sheets been continuously increased, a need for productivity improvement and cost reduction has been greatly highlighted. Thus, studies on technology of omitting the hot-rolled sheet annealing process, acting as a main cause of cost increase, are actively ongoing.
Japanese Patent Laid-Open Publication No. 6-220537 discloses technology for manufacturing non-oriented electrical steel sheets having improved magnetic properties without carrying out hot-rolled sheet annealing. In the technology according to the patent literature, steel containing 1.8 wt % or less of Si+Al is subjected to hot finishing rolling, in which the rolling reduction ratio is limited to 40% or less at a temperature range from an austenite-to-ferrite transformation start temperature +20° C. to an austenite-to-ferrite transformation end temperature −20° C., and the finish rolling deformation rate is limited to at least 50 s−1. According to the disclosure in the patent, if the above conditions are satisfied, deformation resistance caused by the transformation from austenite to ferrite will be lowered so that rolling will be stabilized and magnetic properties will also be enhanced. However, it is expected that the transformation temperature will be lowered due to a low Si content to make grains fine. Also, the disclosed technology is technology for use in manufacturing electrical steel sheets having an iron loss (W15/50) of about 7.00 and is believed to be favorable for improving the shape of hot-rolled sheets rather than for improvements in magnetic properties.
An example of other prior technologies is Japanese Patent Laid-Open Publication No. 2000-297326. In the prior technology, the condition of rolling pass parameter (Z) is limited to improve magnetic properties. However, the rolling pass parameter value should be 16 or less while its fluctuation range should be 2.0 or less, and for this purpose, the deformation rate of hot rolling should be low and the rolling temperature should be increased. However, since the rolling temperature or the deformation rate is determined depending on the ability of a hot rolling mill, it is not easy to apply various conditions. Also, to satisfy the above condition, there is a problem in that a two-step winding must be performed, which comprises winding at a high temperature followed by rewinding.
As another prior technology, Japanese Patent Laid-Open Publication No. 2002-356752 discloses technology of improving by the optimization of base components and an improvement in the manufacturing process without the addition of special elements, in which the size and number of sulfides and nitrides are especially limited. However, measuring the size and number of the sulfides and nitrides produced during the manufacturing process includes many errors, since a very narrow range of observed values are obtained.